A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
The prior art describes an ellipsometric system that enables certain parameters of orthogonally polarized beams to be measured. FIG. 4 shows an example of an ellipsometric sensor (or an ellipsometer) based on the prior art. Illumination radiation from source P is reflected from a structure 30 on a target portion of a substrate W and on its return journey from the substrate, it is linearly polarized along one of the two eigen-polarizations of three beamsplitters that are present in the sensor (the eigen-polarizations being with respect to the x or y direction as shown in FIG. 4). A first beamsplitter 80 sends part of the illumination to an imaging branch; a second beamsplitter 82 sends part of the illumination to a focus branch and a third beamsplitter N-PBS is a non-polarizing beamsplitter that directs part of the beam to a camera CCD. Having passed through the non-polarizing beamsplitter N-PBS, the polarized beam passes through a phase modulator 90 where its ordinary and extraordinary axis have been positioned at 45° with respect to the x and y directions. Subsequently, the beam is divided into its respective x- and y-polarization orientations using a Wollaston prism 50 and impinges on a camera CCD. The relative intensities of the polarized beams are used to determine the relative polarization orientations of the different parts of the beam. From the relative polarization orientations, the effect of the structure 30 on the beam can be determined. From the effect the structure 30 has on the beam, the properties of the structure itself can be determined.
U.S. Pat. No. 5,880,838 (Marx et al.), hereby incorporated in its entirety by reference, also describes the measurement of a structure on a substrate using ellipsometry, wherein the measurement system is called polarization quadrature measurement (PQM). This document describes focusing a polarized beam of light (with TE and TM fields) onto the structure. The TM and TE fields are affected differently by the diffraction off the structure. The TE field can be used as a reference to analyze the phase and amplitude changes in the TM field. The relationship between phases and amplitudes of the TE and TM fields is dependent on the structural parameters (e.g. the depth of a hole or the height of a grating bar or the pitch of a grating) of the structure. By measuring this relationship, therefore, the structural parameters may be determined.
Generally, ellipsometry is the measurement of the state of polarization of scattered light. Ellipsometry measures two parameters: the phase difference (Δ) between two differently polarized beams and an amplitude ratio (tan ψ) of two polarized beams. With these two parameters, any polarization state of a purely polarized beam may be described.
Specifically, if an incident beam has both s and p polarizations, the reflected beam will have reflectance coefficients Rp and Rs. The complex amplitudes of each polarization direction are represented by Ep and Es and are calculated as Rp·p and Rs·s, respectively (the Imaginary parts of the complex amplitude being ignorable when only the reflected beam is considered).
Δ (Delta) is the phase difference between the complex amplitudes Ep and Es as given in equation (1) below.
The intensity of the received beam is proportional to the sum of the amplitudes, taking into account the angle of their relative polarization. For example, if the polarizations of both Ep and Es are aligned in the same orientation, the intensity of the received beam is at a maximum. If the two amplitudes are in orthogonal orientations, they cancel each other out and the intensity is at a minimum. The angle between the two polarization directions (or orientations) is ψ and so the relationship between ψ and Ep and Es is as follows in equation (2).Δ=arg(Ep−Es)  (1)tan ψ=Ep/Es  (2)whereEp=Rp·p  (3)Es=Rs·s  (4)
FIG. 5 shows the relationship between these two parameters. Specifically, FIG. 5 shows the intensity variation in one pixel as a function of phase difference between s and p that is imposed by the phase modulator. I is the intensity of the beam and P is the overall polarization of Ep and Es. Assuming the two amplitudes are the same (i.e. Ep=Es and ψ=45°), the intensity of the overall beam is at a minimum at point x because the polarization directions cancel each other out. At point y, the intensity is at a maximum, indicating that the polarization directions are aligned.
The overall intensity shown in FIG. 5 is modulated, demonstrating that the amplitudes (being the same) cancel each other out to a greater or lesser extent and so the relative phase of the two beams can be monitored as changing accordingly (as dictated by the phase modulator).
The problem with a system such as that shown in FIG. 4 that incorporates a phase modulator is that phase modulators (or phase shifters) have specific disadvantages as listed below.
1. The phase shifts that are applied to the light need to be known exactly because any inaccuracies in these phase shifts will result in the same inaccuracy in Δ. The relationship between intensity and phase must be clear in order for the structure to be accurately determined.
2. Phase modulators are wavelength-dependent, which means that phase modulators have to be recalibrated for each wavelength that is used.
3. With phase modulators, two or more phase shifts are applied to each beam of light at a specific wavelength. The intensities of the differently shifted beams have to be re-measured for each shift, taking significant amounts of time.